Social-engineering the FBI in 1971

In The Burglary: The Discovery of J. Edgar Hoover's Secret FBI, Betty Medsger reveals the long-secret details of the Citizens Commission to Investigate the FBI, an activist group that raided the FBI's offices, retrieving evidence of J Edgar Hoover's criminal program of secret spying. The book is a rollicking history of the confluence of protest, locksport, activism and amateur spycraft. One of its most hilarious moments is the description of the group's social engineering hack on an unpickable lock that they needed to get past in order to get to their target:

As burglars, they used some unusual techniques, ones Davidon enjoyed recalling years later, such as what some of them did in 1970 at a draft board office in Delaware. During their casing, they had noticed that the interior door that opened to the draft board office was always locked. There was no padlock to replace, as they had done at a draft board raid in Philadelphia a few months earlier, and no one in the group was able to pick the lock. The break-in technique they settled on at that office must be unique in the annals of burglary. Several hours before the burglary was to take place, one of them wrote a note and tacked it to the door they wanted to enter: "Please don't lock this door tonight." Sure enough, when the burglars arrived that night, someone had obediently left the door unlocked. The burglars entered the office with ease, stole the Selective Service records, and left. They were so pleased with themselves that one of them proposed leaving a thank-you note on the door. More cautious minds prevailed. Miss Manners be damned, they did not leave a note.